An electro-photographic image forming apparatus can perform a series of processes, such as, for example, electrical charging, image exposure, image development, image transfer, image fixing, and cleaning to form an image on a print medium using toner. Examples of an electro-photographic image forming apparatus may include a laser printer, a copying machine, a multifunction peripheral device, and other like devices.
Such an electro-photographic image forming apparatus can be classified as a multi-path type or a single-path type based on the method used in forming a color image.
A single-path type electro-photographic image forming apparatus can include four photosensitive bodies on which toner images typically of four colors (e.g., yellow (Y), magenta (M), cyan (C), and black (K)) are to be respectively formed. The four photosensitive bodies can be arranged along a line. The toner images of the four colors (i.e., Y, M, C, and K) can be sequentially overlapped on the print medium (or to a transfer belt) to form the full color image while the print medium (or the belt) sequentially passes through the four photosensitive bodies. One of the advantages of the single-path type electro-photographic image forming apparatus is the relatively high printing speed.
The single-path type electro-photographic image forming apparatus can include, in addition to the four photosensitive bodies, a transferring unit having a belt and four transferring rollers disposed to oppose the four photosensitive bodies with the belt disposed therebetween.
As the belt passes between a transferring roller and a photosensitive body, the developer associated with one of the colors is transferred from the photosensitive body to the belt or directly to print medium (e.g., paper) transported by the belt. A full color image is formed when the developers for all four colors are transferred to the print medium in sequence as the belt sequentially passes between the four photosensitive bodies and their associated transfer rollers.
The transferring rollers are conventionally arranged such that their centers are aligned with respect to each other in order to allow the belt to be driven in such a manner that the belt stays relatively flat, and such that they press against the photosensitive bodies with even pressure.
As can be appreciated, an important consideration when forming a color image of multiple colors by overlapping images of individual colors is the alignments or the registration of the individual color images with respect to each other. The color mis-registration, often referred to as the color registration error, can have an adverse impact on the image quality. It is thus desirable to reduce the color registration error for improving the quality of a final color image.